import java.util.*;

/*
 * public class ListNode {
 *   int val;
 *   ListNode next = null;
 *   public ListNode(int val) {
 *     this.val = val;
 *   }
 * }
 */

public class code3 {
    /**
     * 代码中的类名、方法名、参数名已经指定，请勿修改，直接返回方法规定的值即可
     *
     *
     * @param lists ListNode类ArrayList 
     * @return ListNode类
     */
    public ListNode mergeKLists (ArrayList<ListNode> lists) {
        if(lists == null || lists.size() == 0) {
            return null;
        }
        return merge(lists,0,lists.size()-1);
    }

    //分治法的分
    private ListNode merge(ArrayList<ListNode> lists,int left,int right) {
        //边界情况
        if(left >= right) return lists.get(left);

        int mid = left + ((right-left)>>1);
        return mergeKTwoLists(merge(lists,left,mid),merge(lists,mid+1,right));
    }

    private ListNode mergeKTwoLists(ListNode head1,ListNode head2) {
        //特殊情况
        if(head1 == null) return head2;
        if(head2 == null) return head1;

        if(head1.val < head2.val) {
            head1.next = mergeKTwoLists(head1.next,head2);
            return head1;
        } else {
            head2.next = mergeKTwoLists(head1,head2.next);
            return head2;
        }
    }
}